class Solution {
public:
    int reversePairs(vector<int>& aa) {
        vector<long long> a = vector<long long>(aa.size(),0);
        for (int i = 0;i<aa.size();i++) a[i] = aa[i];
        auto a2 = a;
        for (auto& x : a2) x *= 2;
        vector<long long> all = a;
        for (auto x : a2) all.push_back(x);
        sort(all.begin(),all.end());
        all.resize(distance(all.begin(),unique(all.begin(),all.end())));
        map<long long,int> m;
        for (int i = 0;i<all.size();i++)
            m[all[i]] = i;
        for (auto& x : a) x = m[x];
        for (auto& x : a2) x = m[x];
        int N = all.size();

        tree = vector<int>(N * 10,0);
        int ret = 0;
        for (int i = a2.size() - 1;i>= 1;i--) {
            insert(0,0,N-1,a2[i]);
            auto tmp = find(0,0,N-1,0,a[i-1] - 1);
            //cout << tmp << endl;
            ret += tmp;
        }
        return ret;
    }
private:
    vector<int> tree;
    void insert(int p,int l,int r,int x) {
        if (l <= x && x <= r) tree[p]++;
        if (l == r) return;
        int m = (l+r) / 2;
        if (x <= m) insert(p*2+1,l,m,x);
        else insert(p*2+2,m+1,r,x);
    }
    int find(int p,int l,int r,int tl,int tr) {
        if (tl > tr) return 0;
        if (l == tl && r == tr) return tree[p];
        int m = (l + r)/2;
        if (tr <= m) return find(p*2+1,l,m,tl,tr);
        if (tl >= m+1) return find(p*2+2,m+1,r,tl,tr);
        return find(p*2+1,l,m,tl,m) + find(p*2+2,m+1,r,m+1,tr);
    }
};